The following U.S. Pat. Nos. are the most pertinent prior art known to the inventor: 3,583,794, 3,598,107, 3,679,295, 3,832,066.
The present invention generally relates to a device for sensing any movement of an image falling on a photodetector. One important use for such a device is to determine the deviation of the fixation of an eye from a particular point or axis. There are many devices known in the prior art for accomplishing such a task. The most basic one comprises a photodetector on which the image of the eye falls, and a circuit for sensing the change in output of the photodetector which is commensurate with movement of the eye. This sort of device is sensitive only to gross eye movement.
Other prior art devices employ a plurality of photocells, arrayed in pairs and directed to diametrically opposed portions of the limbus of the eye. These systems have a major drawback in that the photocells must be individually directed to the correct portions of the limbus, a process which is time consuming and painstaking. Each photocell requires a separate optical system, thus greatly increasing the cost of the overall system. Furthermore, these types of prior art devices are not adept at sensing blinking of the eyelids, squinting of the eye, or the like. Similar devices, using infrared illumination of the eye, suffer from similar disadvantages.
There are also various ways known in the prior art for processing the signal from the photo detectors. Aside from threshold detectors mentioned in the foregoing, the sum devices have used sample and hold techniques to compare the photodetector signal at any given time with the signal level held from a previous time. A threshold detector is then utilized to sense any differences between the current signal and the held signal, so that significant changes indicative of eye movement may be sensed. These devices are driven by a digital clock to re-sample the signal periodically and update the comparison with the held signal. This form of signal processing has not proven itself to be well suited to sensing small deviations in eye fixation.